1) Field of the Invention
The present invention relates to an image transfer method that employs an electrostatic or electrophotographic imaging forming process.
2) Description of the Related Art
A large number of color documents being handled in the present-day offices demands fast-processing full color printers and copying machines more than ever before. A widely-used typical laser color printer employs what is known as a single drum method. In this method, plural developing devices, which contain developing devices for yellow (Y), magenta (M), cyan (C), and black (Bk), respectively, are arranged in close contact with a single photosensitive element. A toner image of each color is created on the photosensitive element each time the photosensitive element rotates. A full-color toner image is formed when the toner images are sequentially transferred from the photosensitive element to an intermediate transfer element or a recording medium.
There are two methods of transferring the toner images formed on the intermediate transfer element on to the recording medium. In one method, called the intermediate transfer method, the toner images of plural colors are superimposed on the intermediate transfer element, and then a combined color toner image is transferred on to the recording medium in one batch. In the other method, called the direct image transfer method, a color toner image is formed on the recording medium by sequentially transferring a toner image of each color from the photosensitive element to the recording medium. Of the two methods, the direct image transfer method has an advantage of a simple structure, and is cost-effective. In this method, however, when transferring an image by plural times, it is difficult to obtain a stable image forming because conditions such as a friction or an amount of contained moisture of the recording medium may vary. On the other hand, the intermediate transfer method has an advantage of stability of an image quality and handling of various kinds of recording medium because the superimposed image is transferred to the recording medium in one batch.
However, in either of the cases, the photosensitive element has to rotate four times in order to obtain a color image using the four colors, and as a result, the productivity cannot be increased. Therefore, in order to speed up the image forming process, as many photosensitive elements as the number of colors (normally three or four) are provided with their respective developing devices arranged in close contact with corresponding photosensitive elements. A color image is obtained on the recording medium by contacting recording medium from one photosensitive element to another. This method is called the tandem method or the inline method. For example, in Japanese Patent Laid-Open Publication No. S53-74037 (Corresponding U.S. Pat. No. 4,162,843), an image forming apparatus is disclosed in which plural photosensitive elements are provided for speeding up the image forming process in which a transfer sheet is conveyed on a belt-type conveying unit in order to form toner images sequentially on the transfer sheet.
In this case, if the circumferential speed of the photosensitive elements is the same as for a single drum method, a four times higher printing speed can be achieved compared to the single drum method. However, if direct image transfer method described above, in which the toner image is directly transferred from the photosensitive elements to the recording medium, is carried out, there may arise some instability in recording medium transfer or positional deviation in recording medium conveyance. Therefore, a proposal for using what is called a tandem intermediate transfer method, which employs a tandem type intermediate transfer element, is disclosed in Japanese Patent Laid-Open Publication No. S59-192159.
Among recent full-color image forming apparatuses, the most prevalent is a single drum type machine or a tandem type machine that uses an intermediate transfer element, particularly an intermediate transfer belt. However, there are drawbacks of a color image forming method in which the toner images are transferred by plural times on to the intermediate transfer element.
For instance, in a full-color image forming apparatus comprising photosensitive elements, a primary charging device, an image exposing device, developing devices, and four image forming units for the four color toners of cyan, magenta, yellow, and black, and a transfer unit, when transferring images from the third color, the toner that has already been transferred to the intermediate transfer element may be transferred back to the photosensitive element, which is called a reverse transfer.
If the reverse transfer of toner occurs, when recycling spent toner from the cleaner of the photosensitive element at the developing device, it leads to a mixing of different color toners in the developing device. The mixing of colors in the developing device can pose a serious problem when multi-color image formation is involved. Further, the reverse transfer can disrupt the toner image on the intermediate transfer element and eventually lead to a deterioration of image quality.
To cope with the problem, a proposal was disclosed in Japanese Patent Laid-Open Publication No. H9-146334, that the angle of the latent image bearing element with respect to water should be 85° or greater. However, this solution has not sufficiently solved the problem.
According to a study made by the inventors of the present invention, the reverse transfer of the toner from the intermediate transfer element to the photosensitive element mainly takes place in a non-image portion of the photosensitive element because of potential difference.
According to the test conditions of the inventors, the non-image portion of the photosensitive element has an electric potential of −550 volts. In contrast, the electric potential of an image portion where the toner had been developed has a potential difference of about −150 volts and 400 volts. The voltage on the surface of the intermediate transfer element was around +500 volts. The potential difference between the image portion and the surface of the intermediate transfer element in the vicinity of the transfer nip is about 650 volts. In contrast, the potential difference between the non-image portion and the surface of the intermediate transfer element is as large as 1050 volts due to a transfer bias required for transferring the toner image to the intermediate transfer element, leading to discharge of the potential between the non-image portion and the surface of the intermediate transfer element in and around the transfer nip or charge injection into the toner. This discharge of the potential or the charge injection is considered to be a main cause of the reverse transfer. It has been proved that the potential difference between the intermediate transfer element and the surface of the photosensitive element contributed largely to the reverse transfer.
In Japanese Patent Laid-Open Publication No. H5-165383, a proposal to reduce reverse transfer of the non-image portion is disclosed in which a reduction of the reverse transfer is achieved by reducing the potential difference in the image portion and the non-image portion by removing a charge on the surface of the photosensitive element before the transfer nip. FIG. 23A and FIG. 23B illustrate a result of the experiment by the inventors of the present invention, which was carried out to demonstrate the effect on the image of the pre-transfer charge removal by light irradiation; FIG. 23A is an image obtained with the pre-transfer charge removal, and FIG. 23B is an image obtained without the pre-transfer charge removal.
The images in FIG. 23A and FIG. 23B show the effect of the pre-transfer charge removal on clarity of image. This effect appears because of the so-called toner scattering. Toner scattering is caused when the surface of the photosensitive element is exposed to light to remove charge prior to the transfer of the toner image on to the intermediate transfer element and the potential difference between the image portion and the non-image portion is canceled out. This causes the toner image to have charged particles of the same polarity which makes them electrostatically repel each other and scatter before the conveyance of the toner to the intermediate transfer element. Normally, the toner scattering is suppressed because of a higher potential of the non-image portion with respect to the image portion on the photosensitive element. However, charge removal diminishes the suppression effect on the toner scattering.
The inventors of the present invention invented a method for preventing the toner scattering and the reverse transfer. The method removes charge on the photosensitive element by exposing to light the area where the photosensitive element comes in contact with the intermediate transfer element. This method is an innovative method for suppressing the toner scattering and preventing the reverse transfer. However, the method necessitates a use of light-permeable material in the intermediate transfer element, increasing the material-related constraints and thereby making the implementation complicated.